A sequence is not guaranteed to be continuous and gap-free; any 'requirement' you have been given to ensure such behavior is wrong and needs to be discarded. Why do you feel the need to 'replace' missing sequence values? What purpose does that serve? I find no reason to pursue that quest.

WHY is this a 'requirement'? What purpose does it serve? Three of the sequence values you report as 'missing' may not have been generated yet so they really AREN'T missing. What are you planning on doing with these 'missing' sequence numbers? Where will they go? WHY do you 'need' them? There are many unanswered questions with this request and no good reason to pursue it.